PCM-in-water emulsion for solar thermal applications: The effects of emulsifiers and emulsification conditions on thermal performance, stability and rheology characteristics

The aim of this study was to develop a stable water emulsion-based phase change material (PCM), with low viscosity, for solar thermal applications. The effects of different non-ionic emulsifiers, including nine kinds of binary mixtures of Tweens and Spans, on the droplet diameter distribution, the apparent viscosity and the stability of the emulsions, were evaluated. There appeared to be an effective range for both the emulsifier concentration and the dispersed phase PCM content to maintain the stability and the fluidity of the emulsions. The emulsification process also played an important role in controlling the size distribution of the PCM droplets in the emulsions. Thermal analysis by differential scanning calorimetry indicated that the degree of supercooling of the emulsion increased with the droplet size decline and that dispersed nano SiO2particles were effective as a nucleating agent to reduce supercooling. Multiple phase transitions were observed in the melting and the crystallisation processes of the PCM. The rheology characteristics and the long-term storage stability of the emulsions were also investigated and are discussed.